Not sure if this has been discussed already. Both Mewtwo and Lucas' DLC railers used music that is already in SSB4. If further DLC characters are pre-planned, it's quite likely that there's already music for them in the game. After, all it would be weird for, say, Snake to use Nintendo music or for his songs not to be in the game.
Both the trailers for the DLC characters used
Brawl music specifically, so that actually widens the net. Sakurai doesn't seem to be picky about using the newest music for the newest characters.
Now, that doesn't necessarily mean anything. And even if it's true, there's a bunch of characters that it could point to. For example, K.Rool's first boss theme is in the game, and Dixie Kong could probably use Mangrove Cove or something. Doesn't tell us much.
But let's look at some of the games that have music in SSB4 without having a character. I think these are interesting:
Mach Rider (Melee Remix)
The Mysterious Murasame Castle Medley (New Remix)
Lip's Theme (Panel de Pon) (Brawl Remix)
Battle Scene / Final Boss (Golden Sun) (Brawl Remix)
The valedictory elegy (New Remix) [From Baten Kaitos Origins]
Marionation Gear (Brawl Remix)
Attack (Soma Bringer) (Original)
Glory of Heracles (New Remix)
PERFORMANCE (Original) [From Jam with the Band]
Tomorrow's Passion (Original) [From Captain Rainbow]
Style Savvy: Trendsetters (New Remix)
Dillon's Rolling Western: The Last Ranger (Original)
ST01 Roll Out, Wonderful 100 (Original)
These aren't all equally likely, but let's look at them. Both Barbara the Bat and Isaac have had their assist trophies removed, but they actually have new music in this game. Weird. The Murasame Medley would be perfect for a Takamaru trailer, and so would Dillon's theme for a Dillon one.
The Baten Kaitos music is really interesting to me. I assumed Namco owned the series, but the Origins is Nintendo-published. I guess because Nintendo was working closely with Namco anyway, they used the opportunity to make use of the music. I don't think it's going to happen, but it would be neat for a Nintendo-Namco collaboration to have a characer from a Nintendo-Namco collaboration. At least, I think Monolith Soft was still mostly owned by Namco at the time. Soma Bringer's another Monolith Soft game, by the way.
Glory of Heracles is a weird thing to include music from. The series is actually owned by Paon; it's an old Data East series from the late '80s. Actually, I'm not sure if this song even comes from the Nintendo-published game. It doesn't seem to be one of the main themes or one of the battle themes.
Captain Rainbow's another interesting one. If I recall, he was one of the most popular suggestions by Japanese fans. It's like their Wonderful 101, I guess, where the character is popular despite no one having bought the game. Anyway. It would have been a good character to appear as an assist trophy, yet it didn't. But we got the music, so the suggestion didn't go totally unheard. Hm.
You can also note characters that don't have music in the game. On the assist trophy front, there's nothing for Saki Amamiya, for instance. Ray Mk III's assist trophy is gone now, but there's no Custom Robo music either.
(Note: I typed this post up during a class, which is why it's so rambly.)